A piercer used to produce a metal pipe or tube (hereinafter referred to as “pipe”) pierces a round billet as a material and makes it into a hollow shell. The hollow shell is further subjected to hot working using for example an elongator and a mandrel mill and formed into a seamless pipe.
As shown in FIG. 7, the piercer includes a pair of inclined rolls 1 each inclined with respect to a pass line PL, a plug 100, and a mandrel 3 having its front end coupled with the rear end of the plug 100. While turning a billet 50 between the inclined rolls 1 in the circumferential direction, the piercer presses the billet 50 into the plug 100, thus pierces the billet 50 and makes it into a hollow shell 51.
When the billet is pierced with the piercer and formed into the hollow shell, a defect could be formed on the inner surface of the hollow shell (hereinafter referred to as “inner surface defect”) in some cases. The inner surface defects are generated by the following mechanism. During piercing, a fracture due to the Mannesmann effect is caused in the center of the billet upstream of the plug tip end. The fracture due to the Mannesmann effect is subjected to circumferential shear distortion by the inclined rolls and the plug during the piercing. As a result, the fracture due to the Mannesmann effect extends in the circumferential direction to form an inner surface defect.
In order to effectively reduce such inner surface defects caused by the fracture due to the Mannesmann effect, the friction coefficient of the plug surface should be reduced. The reduction in the friction coefficient of the plug surface increases the advancing speed of the billet in the process of piercing. If the advancing speed increases, the rotary forging effect is restricted. Furthermore, such reduction in the friction coefficient can reduce the circumferential shear strain. Therefore, the fracture due to the Mannesmann effect can be prevented from being extended, and the inner surface defects can be restricted.
The reduction in the friction coefficient prevents the plug from being worn or eroded. Therefore, inner surface defects caused by irregularities formed on the plug surface because of the friction or erosion can be prevented.
According to one disclosed technique, a lubricant is injected from an injection hole provided at the plug while the billet is pierced so that the friction coefficient of the plug is reduced. JP 1-180712 A and JP 10-235413 A each disclose a method of piercing while a lubricant is injected from an injection hole provided at the tip end of the plug. The disclosed tip ends of the plugs however each contact the billet. Therefore, in order to inject the lubricant from the injection hole provided at the tip end of the plug, the lubricant must be injected at pressure not less than the deforming resistance of the billet in contact with the tip end. Furthermore, the injection hole could be destroyed by contacting the billet.
JP 51-133167 A discloses a method of injecting a lubricant from a plug without providing the lubricant with additional high pressure. As shown in FIG. 8, the disclosed plug 101 includes a tip end portion 102 having a raised curvature in the axial direction, a cylindrical portion 103 having a fixed outer diameter, and a barrel portion 104 having an outer diameter gradually increased from its front end to its rear end. An injection hole 105 is provided at the part of the barrel portion 104 adjacent to the cylindrical portion 103. When the billet 50 is pierced using the plug 101, a prescribed clearance 60 forms between the inner surface of the billet and the plug surface. During the piercing, although the cylindrical portion 103 deforms to narrow the clearance 60, the clearance 60 allows the injection hole to be kept open and a fixed amount of lubricant to be supplied.
However, the plug 101 could cause an inner surface defect to the billet during the piercing. The injection hole 105 is provided at the part of the barrel portion 104 adjacent to the cylindrical portion 103. Therefore, as shown in FIG. 9, the billet 50 in the process of piercing can contact the upper part of the opening of the injection hole 105. The contact may cause an inner surface defect at the billet 50. Furthermore, if the billet 50 is in contact with the opening of the injection hole 105, the injection hole 105 may be eroded and clogged.
In the plug 101, the lubricant may be solidified to clog the injection hole 105 in some cases. During the piercing, the billet 50 is in contact with a part of the surface of the barrel portion 104 in the vicinity of the injection hole 105. Therefore, the temperature of the opening of the injection hole 105 approximates to the temperature of the billet to attain a high temperature. Therefore, if a glass-based lubricant is used, the lubricant can be solidified in the injection hole 105 and clog the injection hole 105 in some cases.